can i fit a 10" subwoofer under the seats? I know the standard ones are 6.5" and 8.5". What about 10" ?

Sorry, no room for any real subwoofer down there. It's not just the physical depth restriction, but also the amount of airspace BEHIND the woofer. I crammed some 10" Morels in there, but they don't make 'em like that anymore...